In electrochemical etching, the etchant contains an electrolyte, which may not be capable of etching a material to be etched through a chemical reaction (i.e., the etchant does not etch merely through contact with the material). By applying an electric voltage to the etchant between the material and an electrode immersed in the etchant, an electrolytical process, however, is initiated, in which the material is one pole, (e.g., the anode), and the electrode the opposite pole. In the electrolytic process, electric current flows in the etchant, and ions in the etchant react in an etching manner with the material.
One prior art method of etching a disk surface involves the use of a strong acid (e.g., pH 2 hydrochloric acid (HCl)). However, one problem with this method is its isotropic nature (non-directional) in which sidewalls are subject to significant sideways (horizontal) etching, resulting in an undesirable aspect ratio (AR) of about 1. AR is the relationship of the etch depth and the etch width, which may be expressed as:
  AR  =            Z                        (                      X            -            Y                    )                /        2              =                  2        ⁢                                  ⁢        Z                    (                  X          -          Y                )            Where Z is the etch depth, Y is the width before etching, and X is the width after etching.
Etch Width may be expressed as:
      X    -    Y    =            2      AR        *    Z  
An AR of 1 adds twice the etch depth Z to the width of the originally exposed gap area of the disk surface. An AR of 1.5 translates to an added etch width that is 1.33 times the depth Z during the etch process. For example, for a target depth Z of 40 nanometers (nm), an AR of 1 results in 80 nm being added to the starting width, whereas an AR of 1.5 adds 53 nm and an AR of 2 adds 40 nm. FIG. 1 illustrates a difference between an AR of 1 and an AR of 2 for a typical electrochemical, wet-etch process. An embossable layer deposited over a nickel-phosphorous (NiP) layer of a disk substrate forms a width Y prior to a wet-etch process. The recessed area formed in the NiP layer after the etching process has a width X and a depth Z. The acidic etchant typically produces an isotropic effect that undercuts the embossable layer to react with the NiP layer in all directions. For a depth Z of 50 nm, an AR of 1 produces a post-etch width of about 200 nm, while an AR of 2 produces a post-etch width of about 150 nm. Because the width of the recessed area is significantly greater than the depth in typical wet-etch processes (i.e., AR values around 1), achieving high area densities is virtually impossible.
U.S. Pat. No. 6,245,213 to Olsson et al. (hereinafter “Olsson”) describes a low concentration etchant, which etches isotropically in the absence of an electric field, etches anisotropically and at a higher rate, in the presence of the electric field. Olsson discloses that it is possible to etch lines and grooves having greater depth than width, with experiments showing a depth-to-width ratio of 3.5:1 when etching thin copper foil. However, there appears to be limits to how high the depth-to-width may be because the anisotropic nature of the etching process is based mainly on the relatively low concentration of the etchant.
FIG. 2 illustrates a graph showing a theoretical, calculated widening of the initial gap width as a function of the AR for an etch process for a targeted etch depth of 40 nm. As discussed above, an AR value of 1 adds 80 nm to the initial gap width, and decreases (not linearly) as the AR value increases. An AR value of 3 only adds about 25 nm to the initial gap width. An AR value greater than 1 may be desirable in certain manufacturing processes such as in the manufacture of a discrete track recording disk in order to maximizing its recording density.